Threaded fasteners are commonly used with washers interposed between the head of the fastener and the object against which the fastener is tightened. It is known to provide a washer pre-assembled on a threaded fastener. Washers pre-assembled on screws, bolts and the like are commonly referred to as captured washer fasteners. In a known captured washer design, the washer is captured between the head of the screw and an annular ring provided between the head and the end of the thread nearest the head. It is known to form the annular ring by placing a blade die on top of a standard roll thread die. The blade die is pushed into the screw blank during the rolling process, allowing additional material to mushroom out over the top of the blade to form the ring 360° around the shank of the screw.
A disadvantage of the aforementioned captured washer design is that the cross-sectional area of the shank is reduced at the area where the blade die contacts the blank, resulting in higher risk of premature failure as the fastener is tightened at installation. Further, the annular ring is located above the thread run out, therefore increasing the distance between the washer bearing surface and the area of the thread run out. This can be problematic when thin materials are to be captured by the fastener, in that the fastener will run out of screw thread prior to the fastener seating properly with the washer against the material surface.
Thread pitch is commonly referred to as the distance measured from the crest of one thread to the crest of an adjacent thread. In many applications, it is desirable to hold the unthreaded distance between the last thread and the washer (i.e. the thread to washer distance) within one thread pitch distance. When an annular ring is used, the thread to washer distance may be greater than what the specification allows, to provide sufficient space for the formation of the ring after the thread run out. In these applications, an annular ring may not be used.
In situations in which an annular ring can not be used, it is known to use the thread itself to capture the washer on the fastener. During manufacture of the fastener, the washer is captured between the screw thread and the head of the screw, with no annular ring provided. The washer is assembled over the shank prior to the threads being rolled on in the thread rolling operation. Accordingly, the washer inner hole diameter is generally smaller than the major diameter of the screw thread, but slightly larger than the unthreaded shank of the screw. The rolled thread in the non-annular ring design is intended to keep the washer on the part.
The thread rolling process does not allow the major diameter of the thread to be maintained to the very end of the thread, due to the mechanics of the rolling process. In a final pitch or pitches of the thread, the major diameter of the thread decreases steadily. Thread run out is identified as the area of the thread that falls below the specified major diameter for the fastener.
Since the major diameter decreases, it may fall below the diameter of the center opening in the washer. The washer can tilt and drop over the screw thread as the thread fades out. When this happens, the washer can engage the thread, making the part less than desirable for use, and requiring the washer to be freed from the thread before the fastener assembly can be used. It is known for the inner diameter of the washer to lock up on the thread run out, causing the fastener assembly or other part to be unusable. The washer will not seat properly against the mating surface because it no longer sits freely between the head and the thread. In rapid assembly processes, an unusable fastener assembly can be of significant inconvenience.
Accordingly, it is desirable to maintain the specified major diameter and thread pitch through a larger portion of the thread run out, to provide a functional thread that can be maintained close to the washer. It is also desirable to provide a captured washer assembly in which the washer remains free to move as required for proper seating during fastener tightening.